The present invention is related to a method for improving a middle distillate fuel. More specifically, the present invention is related to a method for improving the cetane number of a middle distillate diesel fuel boiling the range of about 160.degree. C. to about 400.degree. C.
The cetane number of a middle distillate diesel fuel is a measure of the ignition quality of the fuel. Fuels having a low cetane number frequently may be characterized by poor cold weather starting, long ignition delays, increased combustion noise and even misfiring.
Methods of improving the cetane number of fuels have taken on increasing importance as the refinery product slate has changed and as the quality of the crude feedstocks has declined. The heavier crudes currently being processed normally produce less virgin middle distillate, and the distillate which is produced tends to be of a lower quality. Moreover, the decline in residual fuel oil consumption often requires that at least a portion of the bottoms produced be upgraded by cracking. This upgraded middle distillate frequently is high in aromatic content, resulting in a low cetane rating. In addition, the rate of consumption of gasoline in many industrial countries has been relatively flat in recent years, while the consumption of diesel fuel and jet fuel has been increasing. To increase jet fuel production, frequently kerosene rich cuts are blended with the jet fuel. Addition of kerosene rich cuts, which have high cetane blending numbers, to the jet fuel necessarily decreases the quantity of kerosene available for use in diesel fuels. These factors have combined to make it increasingly important to maximize the quantity of middle distillate produced and to improve the cetane rating of the middle distillate fuels.
The oxygen content of crudes and of distillate fractions has been reported recently. Typically the oxygenated hydrocarbon content of middle distillate fuels is relatively low, i.e., about 1 weight percent of the fuel.
Fundamental Aspects of Petroleum Geochemistry, Elsevier Publishing Company, New York (1967) reports at page 110 that the oxygen content of crude oil normally is less than 3%, with most of this being fatty acids, naphthenic acids, and naphthenic-aromatic acids. Phenol normally is reported to be present below 0.1 percent, while ketones are present only in trace amounts. At page 151 it is reported that the phenol content of the light oil fraction increases because of cracking.
In a paper entitled "The Nitrogen and Oxygen Compounds in Petroleum" by L. R. Snyder, presented to the American Chemical Society in 1970, the author discloses the types of oxygen compounds present in various distillate fractions. In another paper by L. R. Snyder entitled "Petroleum Nitrogen Compounds and Oxygen Compounds" published in Accounts Chemical Research, Volume 3, 1970, the author presents an analysis of the nitrogen and oxygen contents of several distillates. In these papers the most predominant oxygenated compounds generally are the carboxylic acids. Since these compounds are corrosive, they usually are removed from the distillate, as, for example, by extraction with an alkali. Similarly, phenols are not desired in middle distillates for similar reasons. These compounds also are removed by caustic wash.
Since the oxygenated hydrocarbon content of distillates, excluding carboxylic acids and phenols, is rather low, the oxygenated hydrocarbon content of the middle distillate fuels has been relatively low, i.e., about 1 weight percent of the fuel.
In the past, several approaches have been taken to improve the cetane rating of the middle distillate fuels. One method has involved blending middle distillates having low cetane numbers with other feedstocks having a higher cetane rating. Another method has involved hydrotreating the middle distillate to thereby improve the cetane rating. A third method has involved adding cetane improvers, such as alkyl nitrates, to the distillate to raise the cetane numbers. However, these methods for increasing the cetane number of the fuels have several disadvantages. Blending the middle distillate pool with one or more product streams, such as virgin kerosene fractions, having a higher cetane rating may not be the most profitable disposition of these product streams.
Hydrotreating increasing amounts of middle distillate fuels may not be either desirable or possible at many refineries. Hydrotreating throughput limitations and available sources of hydrogen may limit the amount of middle distillates which economically can be upgraded in this manner. In addition, the cost of operating the hydrotreating facilities may be significant.
Similarly, the use of large amounts of cetane improvers, such as alkyl nitrates, is not desirable, since these additives may be relatively expensive and require balancing cetane improvement against possible loss of desirable fuel stability properties.
Considerable work has been directed in the past at improving the cetane number of middle distillate fuels. U.S. Pat. No. 2,521,698 is directed at the partial oxidation of hydrocarbon fuels to improve their cetane ratings. This patent discloses that the fuel should have a relatively low aromatic ring content and a high paraffinic content.
U.S. Pat. No. 2,912,313 discloses that the addition of both a peroxide and a dihalo compound produces a synergistic increase in the cetane number.
European patent application No. 14,992 discloses that aliphatic polyethers may be used as components of diesel fuel because of their high cetane numbers.
German Pat. No. 3,136,030 discloses that the cetane number of dialkoxyalkanes can be increased by partial oxidation with ozone or hydrogen peroxide.
These methods of increasing the cetane numbers of fuels have not proven completely satisfactory. Oxidation of paraffinic-rich fuel streams may not be desirable, since these streams typically have relatively high initial cetane numbers, and since oxidation has been found to degrade the cetane quality. The use of additives to increase the cetane rating of the fuel may not always be desirable because of the cost of the additives and their adverse effect on fuel storage stability.
Accordingly, it would be desirable to provide a process for improving the cetane number of middle distillate fuels which does not require the use of costly additives or the use of hydrogen. It also would be desirable to provide a process for improving the cetane rating of a middle distillate which does not require the addition of higher quality petroleum streams which have more profitable end uses.
It also would be desirable to provide a reliable process for upgrading the cetane number of middle distillate fuels which requires a relatively low capital investment and relatively little operator attention.
The present invention is directed at a method for improving the cetane number of a middle distillate fuel by selectively converting hydroaromatic and certain aromatic compounds to oxygenated compounds, such as alcohols, ketones, ethers and aldehydes. In one preferred embodiment an aromatic and hydroaromatic rich stream, such as a light cat cycle oil, a light cat heating oil, or a coker liquids stream is selectively oxygenated. The resulting oxygenated stream may be used as a diesel fuel alone, or it may be blended with other petroleum streams to produce a middle distillate stream having an acceptable cetane rating.
In another preferred embodiment, the middle distillate fuel is separated into a first fraction relatively rich in aromatic and hydroaromatic compounds, and into a second fraction relatively rich in paraffinic and cycloparaffinic compounds. The first fraction subsequently is oxygenated. This oxygenated first fraction may be used alone or may be blended with other petroleum streams, such as the second fraction, to produce a middle distillate having an improved cetane number. In still another preferred embodiment, hydroaromatic and aromatic compounds selectively are converted into the corresponding oxygenated compounds in situ.